The present invention relates to a system for measuring the position of a terminal by using cellular communications.
Japanese Patent laid-open (Kokai) No. Hei 7-181242 discloses a system using the CDMA in which each of base stations is synchronous with a GPS by a cellular method and a method for measuring the position of a terminal by observing a receiving timing of a pilot signal received from each of the base stations.
FIGS. 2 and 3 show the technique disclosed in Japanese Patent laid-open (Kokai) No. Hei 7-181242. FIG. 2 illustrates a terminal 24 for receiving reference signals 25, 26 and 27 transmitted from base stations 21, 22 and 23 which are synchronized with a GPS satellite 20. FIG. 3 shows an example of a result of correlation calculation.
By executing a correlation calculation of a specific code pattern on a receiving signal, the terminal 24 can detect a receiving timing of a reference signal transmitted from each of the base stations. In the CDMA, a common pilot signal transmitted from each of the base stations is a signal of a specific pattern. By performing the correlation calculation on the signal, the terminal can detect a receiving timing. Each of the base stations sets reference time by being synchronized with the GPS and transmits a pilot signal at a specific transmission timing of the set system time. The specific transmission timing is called offset time, the information is transmitted via a sync channel and the terminal can freely obtain the information. By calculating a difference between the measured receiving timing and the known transmission timing, the terminal can know the delay time for wave propagation.
FIG. 3 shows an example of the correlation calculation result which is called a delay profile and shows a delay path observed. The lateral axis denotes delay time, that is, the receiving timing corrected by the transmission timing. The unit corresponds to a chip of a spreading code. The vertical axis denotes an output of a correlator. The parts 40 to 43 of large correlation values indicate receiving of a signal, that is, existence of a path. By using the result, relative delay time 44 of a radio wave transmitted from the base station to the terminal can be obtained. The reason why the delay time is expressed in a relative value is because the terminal does not know the absolute time. By multiplying the obtained relative delay time with light velocity, a relative propagation distance difference can be obtained. When the relative propagation distance difference can be obtained with respect to at least three stations, the terminal position can be estimated by hyperbolic position location solution.
Referring now to FIGS. 4 and 5, a near-far problem as another problem of using the cellular communication for measuring a position will be described. In the cellular communication, a receiving power varies depending on the base station-terminal position and a necessary dynamic range exceeds 100 dB. Consequently, a terminal is usually provided with an automatic gain control (AGC) function and the signal strength before A/D conversion is automatically controlled in accordance with the strength of a receiving signal. FIG. 4 shows receiving signals in frequency regions in the case where signals transmitted from three adjacent base stations are received by a terminal. A total receiving power 64 in the band is obtained by adding signals 61, 62 and 63 from the three base stations to a noise 60. The signal 61 from the nearest base station has strong power since the propagation distance is short, so that the signal is dominant in all of the receiving signals. FIG. 5 shows receiving states of two base stations A and B. A noise power 71 occurs due to thermal noise and is constant independent of the position of the terminal. The AGC is set so as to be adapted to the total receiving signal level 70. For example, when the terminal is positioned near the base station A, a signal 72 from the base station A becomes dominant and the AGC operates. Since a digitizing noise 74 fluctuates according to the AGC, its value is high when the terminal is around the base station A. Moreover, at this time, the level 74 of a signal from the base station B is lowered due to a longer propagation distance. The S/I ratio (signal-to-interference ratio)=(signal from B)/(single from A+noise) of the signal from the base station B becomes very low and it is difficult to receive signals from the base station B. In the case of performing wireless positioning, however, even if the terminal is positioned near the base terminal A, it is necessary to discriminate a signal from the base station B.
Another problem will be described. In a mobile communication terminal, a local oscillator of which frequency accuracy is not so high is used for lower price. By being synchronized with the nearest base station, the carrier frequency deviation is reduced (AFC function). There is, however, a frequency difference between the terminal and the base station beyond the limit of the AFC function. Even if there is no fading, perfect synchronization cannot be established. Consequently, the phase of a receiving signal has a slow rotary movement of a few Hz. Even when the user of the terminal is in a stationary state or in a slow movement of about walking speed, the phase of a receiving signal rotates. It is therefore difficult to perform the coherent summation for long time. Since the S/I ratio of a signal from a far base station equivalently deteriorates, it is desired to increase the number of coherent summation times. But, the number of summation times cannot be increased more than a certain degree due to the phenomenon. For example, when there is a terminal having a carrier frequency of 800 MHz and frequency stability after the AFC is 0.01 ppm, the frequency of phase rotation is 8 Hz. When the phase rotation permissible value necessary for the coherent summation is set to 36 degrees or lower, it is understood that the coherent summation can be executed for ⅛× 36/360=0.0125 seconds or shorter. When the coherent summation is performed longer than that, the signal vector rotates and a phenomenon such that the S/I ratio deteriorates occurs.